The severe carburization and decarburization at lower oxygen potentials and internal sulfidation in SO₂ suggest strongly the presence of physical defects, such as micropores and fissures, allowing the passage of gaseous species in the scale, because the solid state diffusion rate of carbon and sulfur in oxide is very low. In the present work sulfur decoration method was applied to detect such defects. Fe-Cr alloys were oxidized at 1173K in impure Ar, H₂-H₂O or O₂. At the end of oxidation run they were sulfidized in H₂S for about 60s at the same temperature. Presence of physical defects in the scale at low oxygen potential was clearly shown by penetration of sulfur into the scale. Such defects allow oxidant gas penetration to the scale/alloy interface and consequently accelerate the oxidation rate, especially the oxidation of Fe-10Cr and -15Cr alloys in Ar.

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Barrier anodic oxide films were formed in an aqueous solution of sodium tungstate on electropolished aluminum sheets of 99.99% purity with and without thermal oxidation. The structure of these films were determined by transmission electron microscopy of ultramicrotomed sections. For the films formed on the thermally oxidized specimens, the development of γ′-alumina islands was observed locally near the middle of normal amorphous barrier oxide layer due to the presence, in the thermal oxide layer, of tiny crystallites of γ-alumina that serve as effective nucleation sites for the growth of γ′-alumina. Except for local regions containing γ′-alumina islands, normal amorphous barrier film formation was observed. No significant differences were observed in the nm/V ratio and the proportion of the outer W⁶⁺-doped layer to the total film thickness between the films formed on the thermally oxidized specimens and those on the electropolished specimens without thermal oxidation. It is concluded that the amorphous thermal oxide layer has little effect, if any, on the ionic transport during barrier anodic film formation on aluminum.

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Corrosion resistances of superheater tube materials of 17-14CuMo, SUS 347H, SUS 310S, 35Cr-45Ni stainless steels have been investigated electrochemically in a molten ternary eutectic of 78mol% Li₂SO₄+13.5mol% K₂SO₄+8.5mol% Na₂SO₄ at 650°C. The corrosion behaviors of these materials strongly depend on the corrosion potential. Bellow a certain potential the protective scale rich in Cr₂O₃ is stable and corrosion rate is low. However, acidic fluxing occurs above a critical potential and results to severe corrosion. This critical potential is affected by material compositions and becomes nobler with increasing the chromium content of steel. SO₃ in coal combution gas acts as an oxidant in alkali sulfate melt and enhances the corrosivity of sulfate melt. According to these measurements, it is concluded that the corrosion resistances to sulfate melt increase in the order 17-14CuMo<SUS 347H<SUS 310S<35Cr-45Ni.

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A newly developed hardfacing Co-bace, carbon-free alloy, Tribaloy, was considered to apply to a continuous decanter centrifuge made of SUS 316 L stainless steel as a material of the screw conveyor flight edge. Stellite, a Co-base alloy containing large amount of carbon, has been used conventionally, and suffered from intergranular corrosion on the stainless steel adjacent to the deposited hardfacing layer. Wear and corrosion resistant characteristics, and the change of the stainless steel overlayed with the hardfacing alloys, were investigated by measuring hardness, anodic polarization curve, optical microstructure, XMA, and EPMA. The following conclusions were obtained: (1) Wear and corrosion resistance of Tribaloy is superior to Stellite in a corrosive environment containing hydrochloric acid and chloride. (2) In contrast with Stellite, Tribaloy does not cause sensitization to a base metal of low carbon type stainless steel even though overlayed by welding with oxyacetylene gas flame as well as TIG welding. (3) Tribaloy has an effect to prevent penetration of carbon from the gas flame to the stainless steel. (4) This material has been applied to a practical centrifuge handling a corrosive slurry, and succeeded to prevent erosion and intergranular corrosion over three years, instead that Stellite had a life of one year or less.

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This review describes the utility of acoustic emission (AE) for the detection and monitoring of corrosion, stress corrosion cracking (SCC) and corrosion fatigue (CF). It surveys laboratory studies and field applications, which have been made to date. Various microprocesses that can produce detectable AE are shown and their relative energy levels, emanated as AE, are discussed in terms of peak amplitudes measured on laboratory specimens by conventional piezoelectric sensor. Practical manners to apply AE as a nondestructive test in engineering structures are presented. All the available information clearly demonstrates the potential usefulness of AE technique.

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Passivity and localized corrosion is discussed using iron, iron-chromium, iron-chromium-nickel alloys and aluminum as examples. A brief description is given of the prevailing ideas regarding the nature of the passive film and the processes by which its protective properties are lost when breakdown of passivity and localized corrosion occurs.

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